Field of the Invention
The present invention relates generally to a control scheme. More particularly the present invention relates to a method and apparatus for reducing a shaft power required when starting up a turbocompressor by manipulating the compressor's antisurge recycle valve.
Background Art
As shown in FIG. 1, a turbocompressor 100, whether axial or centrifugal, is driven by a driver such as a variable speed electric motor 110. A recycle valve 120, used for antisurge protection, is piped in parallel with the compressor 100. An inlet throttling valve 130 may be used for compressor capacity or performance control.
As all those of ordinary skill in this art know, surge is an unstable operating condition of a turbocompressor encountered at generally low flow rates. The surge region is shown in FIG. 2 to the left of the surge limit curve 210. In FIG. 2, Hp is the polytropic head and Q is the volumetric flow rate, both associated with the turbo compressor.
For the purposes of this document, including the claims, the compressor's minimum operating speed is hereby defined as the minimum rotational speed, greater than idle speed, at which the compressor may be operated continuously. The minimum operating speed is defined by the compressor manufacturer. It is generally depicted as the lowest performance curve in a compressor performance map such as shown in FIGS. 2 and 3. Lower speeds, greater than idle speed, are experienced on startup and shutdown, but the compressor is not operated continuously at these speeds. For turbocompressors operated at a constant speed, such as those driven by constant speed electric motors, the minimum operating speed is simply the constant operating rotational speed.
As those of ordinary skill know, the accepted startup procedure for a turbocompressor is to increase the rotational speed of the compressor with the antisurge valve 120 wide open until the compressor reaches the compressor's minimum operating speed (if the compressor is operated at variable speed) or the compressor's operating speed (if the compressor is driven by a constant speed driver). At this point in the startup procedure, the antisurge valve 120 is ramped closed and the compressor's 100 automatic performance control takes control of the compressor's rotational speed, inlet throttling valve 130, or variable guide vanes to control the compressor's 100 capacity.
As is recognized by all those of ordinary skill, this startup procedure provides the most safety for the compressor because surge will be avoided, as depicted in FIG. 3. The compressor's 100 operating point trajectory 320 is shown as a dot-dashed line. Curves of constant compressor rotational speed 310a-310e are shown as solid lines. The curve 310a represents the minimum operating rotational speed, while the curve 310e represents the maximum operating rotational speed. Because the recycle valve 120 is maintained in its fully open position until minimum rotational speed has been achieved, the compressor operating point trajectory 320 tends to give wide berth to the surge limit curve 210 in the region below the minimum rotational speed curve 310a. 
Additional impetuses for startup with the antisurge valve 120 fully open are that the surge limit curve 210 is usually unknown for rotational speeds less than the minimum operating speed, and that pressure and flow sensor signals of reasonable magnitude must be achieved before a valid compressor operating point may be determined. The compressor's operating point must be calculated to compare its location to the surge limit line 210, or surge control line 220 to avoid having the compressor's operating point cross the surge limit line 210. Antisurge control algorithms are described in the Compressor Controls Series 5 Antisurge Control Application Manual, Publication UM5411 rev. 2.8.0 December 2007, herein incorporated in its entirety by reference.
Due to the large flow through the compressor 100 during startup using the above standard procedure, the shaft power required to drive the compressor 100 is large. This results in slower startup and, possibly, tripping of the driver due to power overload.
A gas turbine driver may experience high exhaust gas temperatures during the startup of a turbocompressor. An electric motor driver may trip on thermal overload due to a current being too high for too long a duration.
There is, therefore, a need for an improved control strategy for the startup of turbocompressors to reduce the loading of the compressor while maintaining the compressor flow out of the unstable, surge region.